Thank you. Yeah you hit the nail on its head: it's the environmental conditions that matter. Still it's impressive that a single whole genome has remained large

Thank you. Yeah you hit the nail on its head: it's the environmental conditions that matter. Still it's impressive that a single whole genome has remained largely intact for 100M years or else the cell would not be able to replicate. However for the sequencing of ancient genomes no single genome has to be intact as fragments of many cells can be computationally pieced together into a complete template genome.